The SNP has begun its attempt to break up Great Britain by bringing forward a bill for a referendum on independence. The first minister, Alex Salmond, is due to outline plans to MSPs on Thursday for the next parliamentary session, in which the bill will be formally announced. A spokesman said that a vote on the bill would "place the issue of Scotland's future – and the powers we need to succeed as a nation – at the heart of political and public debate".

Various wishful scenarios declare that the proposed referendum "will never happen", that the "celestial peace of the British Union" will go unperturbed, that the SNP hasn't the majority, or, perhaps less credibly, that the Scottish government is acting beyond its legal remit. But there is good political reason to think that the referendum will happen and its result is by no means certain.

First, there's that annoying problem of democracy. Iain Gray will need to face-up on Thursday and present a coherent response. All of the opposition parties in Scotland share the difficult prospect of opposing the people's right to have their say. Let's be clear, there are many more people who want a poll on independence than want independence itself or who would vote for the SNP. Support for a referendum on independence is vast, ranging from between 60% and 80% of the Scottish public in recent opinion polls.

At its heart the referendum offers the prospect of power to change beyond constitutional paternalism. As one wag put it when firearms legislation was being proposed at Holyrood, "with devolution you get to ban air-rifles, with independence you can cancel Trident". Democracy for Scotland through referenda is both means and ends. Then there's the Cameron-effect. The Tories may be riding high in the opinion polls in England but in Scotland they are flat-lining worse than Kevin Bacon and Keifer Sutherland ever did. In the immediate future, Scottish politics will at least be more interesting than Westminster. Brown may have consigned himself to political oblivion, but in Stirling and Dundee it's not Brown v Cameron, it's Cameron v Salmond, and the Cameronians are not half as clever as they think they are.

Polls have shown if a Cameron Tory cabinet forms support for independence jumps 25% . Even at its poorest ratings for a year (a YouGov poll shows support for independence at only 28%) the "Cameron Effect" could easily be a stepping stone to another Yes vote. The problem for Unionist politicians rubbing their hands in glee at this post-Megrahi nadir, is that the survey also showed a seven-point SNP lead over Labour in the first-past-the-post constituency vote and a four-point lead in the poll for list MSPs. People may think MacAskill wrong but they don't doubt his integrity.

In a cruel paradox for Labour a referendum offers a way out from Tory public spending cuts, and it's as campaign allies that Labour will be playing up the threat of the Tories' plans.

The third and most compelling reason that may yet swing the issue is the collapse in credibility of Westminster and politics in general. Groups like Vote for a Change are mushrooming and likely to reinforce the idea of a failed political class mired in corruption, back-handers and moat-expenses. Holyrood is not immune from petty dishonesty, but it isn't swamped with the same association of base venality that dogs Westminster. For many its not a gigantic leap to suggest that it's the British state itself that needs disbanding. From Tomlinson to Peace Camp, from the surveillance culture to ID cards the prospect of an authoritarian state is an anathema to most Scots.

The unionist coalition may well play the fear card that was the tried and tested political tactic to oppose first devolution itself, and then a nationalist administration. The Megrahi decision will be portrayed as botched foreign policy, the financial meltdown as a sort of 21st century Darien, and the incoming Tories will no doubt try and portray David Cameron as a sort of Bambi in Tails.

The very institutions that could hold Britain together as an idea have been picked apart, privatised, sold off or dismantled by two decades of neo-liberal politicians who can hardly now appeal to the NHS, the Post Office or a common media voice as indicators of a common future, never mind a shared past. If you place so little value in these institutions then don't rely on them to tell your political story.

The Megrahi case has sent many English commentators into a sort of spasm of resentment, anger and confusion, Fraser Nelson perhaps taking the prize for wilful stupidity. This sort of coverage is hardly a revelation but come the referendum the combination of the sort of harsh surveillance state established under Blair-Brown with a return to reckless unashamed Tory sado-monetarism may be too much for a nation with a ready alternative.

Conventional wisdom is that the Megrahi case has sent independence off-track, shattering confidence in self-determination just as the banking collapse was supposed to. This didn't happen and next year's referendum might just confirm the inevitable.

Editor's note: The headline of this article was changed at 09.38 on 2 September 2009